The present invention is related to an inrush current protection circuit for preventing an inrush current from generation in a power supply.
As a DC-powered electric appliance such as a desktop computer or a laptop computer intends to employ an AC electric power source as its power source, a power adapter is required to perform power conversion from AC electric power into DC electric power. Referring to FIG. 1, there is shown an electric power system including a power adapter 11 for converting the AC electric power 12 into DC electric power to power an electric appliance 10. None the less, a problem is prevalent among the prior electric power systems, that is, when the electric appliance 10 is powered on or the power adapter 11 is in the initial state that the plug of the power adapter 11 (not shown) is just plugged into the outlet of the AC electric power (not shown), an excessive inrush current will be very likely generated to cause sparks on the outlet or switch. More seriously, other electric appliances in use will be interfered and the quality of the power source will be deteriorated, and further the sparks would cause unpredictable damages. For the purpose of preventing the occurrence of inrush current, a protection circuit is pioneered as shown in FIG. 2.
FIG. 2 shows a prior electric power system with an inrush current protection circuit incorporated therein. In FIG. 2, the rectification circuit 111, the energy-storage capacitor 112 and the main body 113 form a power adapter 11 as indicated in FIG. 1. The negative temperature coefficient thermistor 21 located at one side of the AC electric power is used to regulate the probable inrush current in the electric power system. The way to regulate the probable inrush current in an electric power system by the negative temperature coefficient thermistor 21 is to take advantage of the property of the negative temperature coefficient thermistor 21 that it has a high resistance under a low temperature and has a low resistance under a high temperature. When the electric appliance 10 is just powered on or the plug of the power adapter 11 is instantaneously plugged into the outlet of the AC power 12, because the negative temperature coefficient thermistor 21 has a lower temperature, a higher resistance is provided at one side of the AC electric power to suppress the inrush current in the electric power system. After a period of time that the electric power system is continuously operating, the temperature of the negative temperature coefficient thermistor 21 is lowered and its resistance is reduced, and the whole system enters into a normal operation mode. However, the electric power system of FIG. 2 is disadvantageous in terms of great power loss and can not be brought into full play in the hot swap stage.
Referring to FIG. 3, another type of prior inrush current protection circuit is shown. The inrush current protection circuit of FIG. 3 makes use of two switches 31 and 32 to regulate the probable inrush current. It can be clearly seen from FIG. 3 that the switches 31 and 32 are installed at the positive terminals of the rectification circuit 111 and the energy-storage capacitor 112, and controlled by means of the voltage signals from the high-voltage terminal 1131 of the main body 113. As a result, the specifications of the switches 31 and 32 have to match with the high voltage-resistant requirements (normally the voltage-resistant requirement is set above 600 volts). Consequently the circuit structure of FIG. 3 is deficient in terms of high circuit design complexity and high cost of the individual circuit elements. It is therefore a major object of the present invention to develop an inrush current protection circuit that can obviate the aforementioned drawbacks encountered by the prior inrush current protection circuit for a power supply.
The present invention essentially addresses an inrush current protection circuit for a power supply. According to the present invention, the power supply includes a rectification circuit and is connected to a main body of the power supply through an energy-storage capacitor. The power supply possesses a reference voltage with a setup time greater than a threshold value. The inrush current protection circuit for a power supply in accordance with a preferred embodiment of the present invention includes a current limiting resistor coupled in series between a negative terminal of the rectification circuit and a ground terminal of the energy-storage capacitor for preventing an inrush current from generation at an instant that the power supply is powered on, and a switch coupled in parallel with the current limiting resistor which switches from open state to short state after the reference voltage is set up, whereby bypass the current limiting resistor.
Another respect of the present invention comprehends an inrush current protection circuit for an electric appliance. The electric appliance includes a rectification circuit and is connected to a main body of the electric appliance through an energy-storage capacitor. The electric appliance is provided with a reference voltage with a setup time greater than a threshold value. The inrush current protection circuit includes a current limiting resistor coupled in series between a negative terminal of the rectification circuit and a ground terminal of the energy-storage capacitor for preventing an inrush current from generation at an instant that the electric appliance is powered on, and a switch coupled in parallel with the current limiting resistor which switches from open state to short state after the reference voltage is set up, whereby bypass the current limiting resistor.
The inrush current protection circuit in accordance with a preferred embodiment of the present invention further includes a capacitor coupled in series between a reference voltage input terminal of the switch and a negative terminal of the rectification circuit for increasing the setup time of the reference voltage, a resistor coupled in series between the reference voltage input terminal of the switch and the reference voltage for regulating a charging current of the capacitor as below a predetermined current level, and a PNP-type bipolar junction transistor with a base coupled to the reference voltage, an emitter coupled to the reference voltage input terminal of the switch and a collector coupled to a negative terminal of the rectification circuit for accelerating the charging operation of the capacitor as the reference voltage is dropped to a predetermined voltage level.
Further, the inrush current protection circuit of the present invention includes a zener diode coupled in series between a reference voltage input terminal of the switch and a negative terminal of the rectification circuit for limiting the voltage across the switch.
In a preferable aspect of the present invention, the switch is formed from a metal-oxide-semiconductor field-effect transistor (MOSFET), and the threshold value is exactly the chargeup time of the energy-storage capacitor.